Impedance matching generator



Dec. 1, 1953 Filed May 14, 1949 E- MITTELMANN IMPEDANCE MATCHINGGENERATOR 2 Sheets-Sheet 1 h a Z 7 0 ions ENTOR.

' mwzmm Lona IMPEDANCE Dec. 1, 1953 E. MITTELMANN IMPEDANCE MATCHINGGENERATOR 2 Sheets-Sheet 2 Filed May 14, 1949 Patented Dec. 1, 1953IMPEDANCE MATCHING GENERATOR Eugene Mittelmann, Chicago, Ill., assignorto- Mittelmann Electronics Corporation, Chicago, 11]., a corporation ofIllinois Application May 14, 1949, Serial No. 93,328

21 Claims.

that the impedance of the oscillator should be matched to that of theload. In high frequency heating, the load reflected to the oscillatorfrom the heater varies with the material placed in the heater field andalso may vary during any one operation as the temperature of thematerial changes. Heretofore it has been common practice to vary thecoupling between the oscillator and the heater in order to retain thematch of impedance therebetween. There are, however,

times where it is undesirable or impossible to vary the coupling betweenthe oscillator and its load, and yet it is desirable to retain animpedance match therebetween for the optimum transfer of energy.

An important object of this invention is to produce a method of andmeans for varying the effective impedance of an oscillator or agenerator so that it will be continually matched to a changing load.

The equivalent dynamic impedance of a vacuum tube oscillator is not aconstant which is characteristic for the tube used but is a variablemagnitude which is a function of the operating parameters. As a generalrule, high ratios of plate voltage to plate current will correspond tohigh values of the equivalent generator impedance and correspondinglylow values of the same ratio will indicate a low equivalent impedance.

If the load impedance changes, toward lower values, the plate current ofthe generator will increase, and conversely will decrease if the loadimpedance increases. If the equivalent generator impedance is to bechanged, as contemplated by this invention to match a decreasing loadimpedance, it is apparent that the plate current must rise and at thesame time the plate voltage must decrease in order to obtain a new andlower value of generator impedance. Conversely, to match an increasingimpedance the plate current must be lowered and the plate voltageraised. In order to increase the plate current with a diminishing platesupply voltage, it is necessary to increase the grid swing, i. e., toincrease the angle of plate current flow,

Further explanation of this invention will be carried out in conjunctionwith the accompanying drawings in which:

Fig. 1 is a schematic diagram of a simple circuit embodying myinvention;

Fig. 2 shows a variety of curves representing the operatingcharacteristics of an oscillator tube with constant power output andchanging load impedance; and

Fig. 3 is a schematic diagram of a further embodiment of the presentinvention.

In order to best illustrate the phenomena involved, the operatingcharacteristics for constant power output of an oscillator tube with achanging load impedance have been calculated and plotted in Fig. 2. Thevalues illustrated are those for a type 498 tube and constant poweroutput of 33 kw. From an inspection of the curves, it may be seen thatthe D. 0. plate voltage increases with increasing impedance, therelationship between the D. 0. plate voltage and load impedance beingsubstantially linear in the region investigated. The grid swing, whichis the peak value of the radio frequency grid voltage, decreases withincreasing load impedance. The plate current also decreases withincreasing load impedance. The D. C. grid bias increases with increasingload impedance but varies only slightly over nearly half of the regioninvestigated. It may also be noted that the efilciency increases withincreasing load impedance is the region investigated but changes onlyfrom 66% to 72.5% from 1200 ohms to 2500 ohms.

Inspection of the curves reveals that if a constant power output is tobe maintained within a certain load impedance range, that peak gridvoltage and D. C. plate voltage must vary inversely.

In Fig. l is shown a circuit embodying my invention which maintainsconstant power output by increasing the plate current at a diminishingplate voltage by increasing the grid swing, i. e., by increasing theangle of plate current flow.

The oscillator is shown as being supplied with power from three supplylines 2, 4 and 6 connected to a power transformer, the primary 8 ofwhich is delta connected. It is contemplated, however, that a singlephase supply source could be used. The secondary I0 is Y connected withthe legs of the transformer connected to wires I 2, I4, Hi, the wiressupplying a three-phase full wave rectifier. The rectifier includesthree diodes l8, 2|] and 22, which may be of the gas filled type andhave their anodes connected in parallel to a. B supply line 24. Thecathodes are connected respectively to the anodes of grid con trolledtubes 26, 28 and 30 which are preferably gas filled, and are alsoconnected to the wires I2, I4 and I6 leading from the secondary of thepower transformer. The cathodes of these three triodes are connected inparallel to provide a 33+ potential which is preferably grounded as at32 to preclude the possibility of a high D. C. voltage appearing on theheater. The grids of the triodes are connected to symmetrically disposedpoints 34, 36 and 38 of a phase shifting network 40 which includes the Yconnected secondary 42, the delta connected primary 43 of which isconnected to the supply lines 2, 4 and 6. The phase relation between thegrids and plates is thus predetermined to help control the firing timeof the triode. The midpoint of the Y connected secondary 42 is connectedto a Wire 44, the purpose of which will be explained in detail later.

The oscillator itself includes an electron tube 46 which is illustratedas a triode having its plate connected to a tuned plate circuitincluding an inductance 48 and capacitance 50 connected in parallel. Theend of the tuned circuit opposite the plate is connected to ground as at52 to supply B+ voltage. The grounded end of the plate tank circuit isalso connected to the cathode of the oscillator tube 46 by a condenser54. A reactive heater B, which may be a dielectric or an inductionheater, is supplied with power from the oscillator tank circuit bydirect coupling including a capacitor 58.

The feed back circuit of the oscillator includes a capacitor 80 and aninductance 62, connected between the plate and cathode, the junctionpoint between them being coupled by a capacitor 64 to the grid of thetube 46. The grid is also connected to the cathode by an inductance 66and a resistor 68 connected in series. The cathode is connected to the8* supply line 24' through a resistor to complete the D. C. platecircuit. The feed back inductance 62 comprises the primary of atransformer having a center tapped secondary 12. The ends of thesecondary are connected to the plates of a pair of triode electron tubesI4 and 16, the cathodes of which are connected in parallel to thecathode of the oscillator tube 46. The plate circuits of the tubes 14and 16 are completed through a current limiting resistor 18 connectedbetween the center tap of the secondary I2 and the cathodes. To the Bsupply line is connected a potentiometer I8 which is in turn connectedto series resistors 80 and 82,

'the latter of which is grounded. The grids of the tubes I4 and I5 areparallel connected to the tap 84 on the potentiometer T8 to supply D. C.grid'bias.

To the grounded end of the plate tank circuit is connected an inductance86 which is inductively coupled to the inductance 48. The other end ofthe inductance 86 is connected to a rectifier 88 and the return circuitfrom the rectifier is completed by a resistor 90. The junction be tweenthe rectifier 83 and resistor 90 is connected to the previouslymentioned wire 44 and thence indirectly to the grids of the gridcontrolled rec tifier in the power supply.

It is apparent that for any given value of capacity 60 the feed backvoltage is determined by the magnitude of the inductance 62. inductance62 is inductively coupled to the secondary 12, its magnitude will be inlarge measure determined by the current carried by the secondary I2,having a maximum value when the current in the secondary is at aminimum, d a

As the i such as might be used in a commerc .l tion. Power is suppliedthrough three-phase additional voltage drop across the resistor I0connected to the B- supply. The increased voltage drop across theresistor I0 applies an increased negative bias to the grids of the tubesI4 and I6, reducing the current through the secondary I2,

" and as a result increasing the effective inductance of the inductance62. The feed back voltage to the tube 46 is thus increased, yielding anincreased angle of iiow of the plate current. To reduce the generatorimpedance to match the new load impedance while maintaining the poweroutput constant, it is necessary to reduce the plate supply voltagealong with the increasing plate current as a function of the increasedgrid swing. In the oscillator shown, the grid swing is directlyproportional to the R. F. tank voltage, and as a consequence, a voltagederived from the tank inductance 48 will be proportional to the voltageacross the inductance 62. Such a voltage is derived from the tankinductance 48 by the inductance 88 coupled to it and is rectified by therectifier 88 to develop a voltage across the resistor 90 which isinjected with a negative polarity by means of the wire 44 and the phaseshifter 40 to the grids of the grid control rectifier 5 tube 26, 2-8 and30. Due to the decreased grid bias. these tubes now fire a lesser amountand the plate voltage is reduced to conform to the requirementillustrated in Fig. 2.

When the plate voltage decreases, voltage across the tapped oif portionof the potentiometer it becomes less positive, decreasing the currentthrough the tubes 14 and "H5. The consequent decreased current throughthe secondary 5'2 thus raises the eifective inductance of the inwardplate current reduction due decreased plate voltage.

In Fig. 3 is shown another embodiment of the current invention, thisembodiment being one applicasupply line having the wires H12, I04 andIE5. To

these wires are connected the series coils H38 of an overload circuitbreaker having movable switch arms I I0. On the load side of tiebreakers is a push button switch I I2 a holding coil H4 connected on theload of the push button switch. The push button switch H2 preferably ofthe type having two buttons, one of which is pushed to make theconnection which. by virtue of the holding coil Ii until the other pushbutton is pushed to break connection. Wires lead from the push buttonswitch to the delta connected prir a power transformer, the Y connectedsecondary III! having'its legs connected to I29, I22 and H4. These wiresare in turn connected to the center taps of the secondaries I28 and I offilament transformers, the primaries of which are not shown. The ends ofthese second aries a're'connected to the heater-cathodes of dioderectifier tubes I32, I34 and :36 which are preferably of the gas filledtype. The wires I20, I22 and I24 are also connected to the anodes ofgrid controlled rectifier tubes 38, I and I42 which are also preferablythe gas filled type. The heater-cathodes of the grid controlledrectifiers are connected in parallel to the secondary I40 of a filamenttransformer, the primary of which is not shown. A center tap of thesecondary I40 is connected through an ammeter I43 and a resistor I44 toground, thus providing a grounded 13+ voltage supply. A tap on theresistor 44 leads to the actuating coil of an over load relay I46, theother end of which is grounded. The grids of the controlled rectifiertubes I38, I40 and M2 are grounded through capacitors I48, I50 and I52and are connected through resistors I54, I56 and I58 to symmetricallydisposed points on a three-phase shifting network I55 including the Yconnected secondary I51 of a transformer, the delta connected primaryI55 of which is connected in parallel with the primary II6 of the powertransformer. The center point of the Y connected secondary I55 isconnected to a wire I60.

The generator comprises an oscillator tube Hi2 which may have a cathodeI64 connected through a capacitor I66 to ground and also connectedthrough a resistor I68 to the B- supply of the rectifier. The filamentleads of the oscillator tube I62 are respectively connected to afilament transformer, the connections being in dicated as X-I and X-2.The filament leads are also connected through capacitors H and I12 tothe cathode I 64 and to ground. The plate llll of the oscillator tube isconnected to a tuned sircuit comprising an inductance I16 with threeparallel capacitors I18 connected in parallel thereto. A singlecapacitor could of course be used rather than the three shown here. butit is preferable to use a plurality of standard capacitors rather thanrequiring one of non-standard construction. It will be noted later thatother circuit elements are arranged in plural numbers for a like reasonand also for more efficient heat dissipation. The opposite end of thetuned circuit is connected through a current limiting resistor I80,paralleled by a capacitor I82, to ground to complete the B-+ supply forthe oscillator plate circuit.

A feed back circuit for the oscillator tube I62 includes a pair ofcapacitors i8 and E85, the latter being tunable, connected in parallelto the plate I14 and an inductance I83 which is the primary of atransformer. The other end of the inductance m8 is connected through acapacitor I 90 to ground and through a plurality of resistors I92, oneof which is variable in series with a parallel combination of acapacitor 29 and an ammeter I96 to the B supply from the rectifier. Thejunction between the capacitors I34 and I86 and the inductance I88 isconnected to the control grid I98 of the oscillator tube by a couplingcapacitor 260. The control grid is also connected through seriesinductances 202 to the junction between the inductance I88 and resistors92 to provide D. C. grid bias and current for the oscillator tube I62.To the inductance IE8 is coupled an inductance 204 which is the centertapped secondary of the transformer having I68 for a primary. The endsof the secondary 2M are connected to the plates of a pair of electronictubes 200 and 208 which for illustrative purposes are shown as beingtriodes. The center tap of the secondary 204 is connected through aresistor 2I0 to the cathodes of the tubes 2% and 2% to complete theplate circuit and also to the cathode of the oscillator tube I'M. Thecathodes of the tubes 206 and 2&8 to complete the plate circuit and alsoto the cathode of the oscillator tube 514.

6 The cathodes or the tubes 206 and 208 are also connected through acapacitor M2 to a wire 2I4 to which the grids of the tubes 206 and 208are also connected in paralleli The wire 2M provides D. C. bias for thegrids from a voltage divider comprising a tapped potentiometer 2E6 and aplurality of resistors 2I8 connected between the B- supply and ground.

A heater 220 is shown diagrammatically as it may be of any desirabletype. The heater is grounded as at 222 and is coupled as by aninductance 224 to the tuned circuit inductance I16 in the plate circuitof the oscillator tube I52 to receive high frequency power therefrom.

A rectifier grid controlling circuit is coupled to the inductance I16 byan inductance 226 rounded at one end as at 228 and having a resistor23%? and rectifier 2 2 connected in series with it. The voltagedeveloped across the resistor 230 is applied with negative polarity to afilter 232 comprising inductances 234 and capacitors 236. From thisfilter the potential is carried by a wire 238 to a junction point 2st ofa network 242 including resistors 24s, 2526, 2st, 250 and 252 in aclosed series circuit. A step down transformer 25 5 provides volts froma pair of the three-phase supply lines I92, I64 and I36 so that anordinary 110 volt input power transformer may be used for a purpose tobe described. The primary 25E; of such a 110 volt transformer may beconnected directly to the secondary or" the transformer 256. A filamentwinding 262 is connected to the heater cathode of a full wave rectifiertube 26 the plates of which are connected to the extremities of asecondary coil 2&6. A center tap on the winding is connected to ajunction point 253 to provide negative potential to one end of thenetwork 242. The filament winding 252 is connected to another junctionpoint 219 at the opposite end of the network 242 to provide positivepotential at that point, a filter comprising capacitors 212 andinductance 214 being connected across the rectified voltage supplied toeliminate ripple. The resistor 248 of the network 252 is paralleled bythe switch 216 of the overload relay lee, and a movable tap 218 on theresistor 2 is connected to the previously mentioned wire and from thencethrough the phase shifting network I55 and grid resistors I56, I56 and258 to the grids of the controlled rectifier tubes I38, I 33 and I42.

When an article to be heated is placed in the reactive heater 229, thepush button switch 2I2 may be actuated by pushing its associated pushbutton. The holding coil us is then energized to keep the switch I I2closed until such time as 'it is manually opened by an associated pushbutton. As the switch H2 is closed power is supplied to all of the powersupply transformers in the apparatus and as soon as the filaments haveheated up the generator including the oscillator tube I62 begins tooscillate to supply power to the heater. Assuming the generator and loadto have been matched, the impedance match between the load and generatorwill change as the article being heated changes with temperature orthere will be an initial mismatch if the load inserted was not of thesame impedance as the last previous load. If the load impedancedecreases the plate current will increase suddenly, thus increasing thevoltage drop across the resistor I53 connected in the oscillator cathodecircuit. As the potential across the resistor I68 increases it providesan increased negative bias for the grids of the control tubes 2135 and208, thus decreasing the current carried by these tubes and by thesecondary 264. As was explained with regard to the inductances and 12 inFig. l, the decreased current in the sec ondary increases the effectiveinductance of the primary, in this case I88, thus increasing the feedback voltage to the oscillator tube 152 yielding an increased angle offlow of the plate current. The plate current is thus increased be---yond its original value. To decrease the equivalent generator impedanceit is necessary at the same time to reduce the plate supply voltage as afunction of the increased grid swing. The: voltage induced in theinductance 222%; is proportional to that in the inductance l'iG of theoscillator plate tank circuit and is hence proportional to the gridswing. The alternating current voltage so induced is rectified by therectifier 232 to produce a direct current voltage across the resistor2%, this last voltage being applied with negative polarity to thejunction point 2 3 of the network 2% by means of the filter circuit 232and the wire 238.

The grids of the controlled rectifier tubes {38, I40 and 142 aremaintained a predetermined an-- gle out or phase with the plates by theshifter IE5 which is coupled by means of the transformer secondary andprimary 5 to the same power supply source as the power transformerprimary and secondary i and I it, to control the amount of current byrectifier tubes. Further control is had by means of the adjustable tap13 on the network 2:32 which determines the grid bias of the controlledrectifier tubes. The voltage picked by the tap H8 is made more negativeby the negative voltage applied at Hi due to the decreased lot Iimpedance and this reduces the grid voltage of the controlled rectifiertubes we, l and i to reduce the rectified voltage, thus decrees he; theequivalent generator impedance to match that of the load.

As the rectified plate voltage decreases, a lesser potential appearsacross the voltage divider comprising the potentiometer file andresistor 2H! and consequently the voltage tapped off across thepotentiometer 2N5 becomes less positive, thus decreasing the currentthrough the tubes and tilt and consequently the current through thesecondary 2st. Accordingly, the effective inductance of the inductanceHi3 is raised to increase the feed back voltage and compensate for thetendency toward a plate cur-- rent reduction due to the reduction ofplate voltage.

If the plate current becomes too high at any time the cathode current ofthe rectifier will be increased sufficiently to cause the overload.relay coil ME to actuate its associated switch to lower the potentialapplied to the grids of the controlled rectifier tube 138, hill and M2and thus cause these tubes to fire at a later time in each cycle andconsequently reduce the plate current to a safe level. This platecurrent may at all times be read on the ammeter M2 and in like mannerthe grid current of the oscillator tube I62, which is controllable bythe variability of one of the resistors I92, may b read on the ammeter[$16. Other safety devices which may be incorporated include the currentlimiting re" sistor 210.

Although for purposes of illustration I have shown and described certainpreferred embodiments of my invention, 1 do not intend. to be 8 limitedthereby but only .by the spirit and scope of the appended claims.

The invention is hereby claimed as follows:

1. The method of varying the impedance of a high frequency generatorhaving plate current and output voltage parameters to match .a varyingimpedance load which comprises varying both the plate current and directcurrent plate supply voltage of the generator relatively inversely inresponse to change in one of said parameters.

2. The method of varying the impedance of a high frequency electronictube generator having plate current and output voltage parameters tomatch a varying impedance load while maintaining a constant power inputto the load which comprises varying the direct current plate supplyvoltage of the generator positively relative to the variation in loadimpedance, varying the generator plate current inversely relative to thechange in load impedance and varying the angle .of plate current flow ofthe electronic tube inversely relative to the varying impedance, thegenerator variations being controlled by a change in said parameters.

3. The method of varying the impedance .of a high frequency electronictube generator to match a varying impedance load while maintaining aconstant power input to the load which comprises deriving an oscillatorfeedback voltage L varying inversely with the impedance of the load,

utilizing said voltage to vary the angle of plate current flow of theelectronic tube inversely relative to the load impedance, deriving avoltage proportional to the feedback voltage, and utiliz- 1 ing thislast named voltage to vary the direct current plate supply voltage inaccordance with the change in load impedance.

4. In a high frequency generator supplying constant power to a heater,means for continuously varying the impedance of the generator to match avarying load impedance comprising an oscillator having output and inputcircuits with plate current and output voltage parameters, means forvarying both the oscillator direct current plate supply voltage andplate current relatively inversely, said last named means includingmeans for varying the angle of plate current flow in response to achange in said parameters.

5. In a high frequency oscillator having output and input circuits withplate current and output voltage parameters, means for supplyingconstant power to a variable load, said means including a high frequencyfeedback circuit having a plurality of reactance elements, and means forchanging the value of at least one of said reactance elements to changethe feedback ratio in response to change in one of said parameters.

6. In a high frequency generator supplying power to a heater, means forcontinuously varying the impedance of the generator to match a varyingload impedance comprising an oscillator having output and input circuitswith plate current and output voltage parameters, means for varying theoscillator plate current inversely relative to the changing loadimpedance, means for varying the oscillator direct current plate supplyvoltage positively relative to said changing load impedance, said lasttwo means including means responsive to a change in parameters.

7. In a high frequency generator supplying power to a heater, means forcontinuously varying the impedance of the generator to match a varyingload impedance comprising an oscillator,

means for deriving a high frequency oscillator feedback voltage themagnitude of which varies inversely with the changing load impedance,means for varying the plate current in accordance with said feedbackvoltage, and means for varying the oscillator direct current platesupply voltage inversely relatively to said feedback voltage.

8. Impedance matching means as defined in claim 7 in which the means forvarying the oscillator direct current plate supply voltage inverselyrelative to the feedback voltage includes mean or de v ng a co trol vola e o rtio to the feedback voltage, and means for utilizing said controlvoltage to vary said direct current plate supply voltage inverselyrelatively to said feedback voltage and directly relative to saidchanging impedance.

9. In a high frequency generator, means for supplying constant power toa variable impedance load, said means comprising an oscillator havinginput and output circuits with plate current and output voltageparameters, a feedback circuit for said oscillator, energy absorptionmeans in said feedback circuit and means responsive to change in one ofsaid parameters to vary the energy absorbed from said feedback circuitwhereby to vary the feedback voltage.

10. In a high frequency generator supplying power to a heater, means forcontinuously varying the impedance of the generator to match a varyingload impedance comprising an oscillator, means for concurrently varyingthe oscillator plate voltage and plate current relatively inversely,said means including a high frequency oscillator feedback circuit, avariable reactive element in said feedback circuit, and means forvarying the reactance of said reactive element inversely relative tosaid changing load impedance.

ll. Impedance matching apparatus as defined in claim 10 in which thevariable reactive element is a variable inductance.

12. In a high frequency generator supplying power to a heater, means forcontinuously matching the generator to a varying load impedancecomprising an oscillator, means for varying the plate voltage of saidoscillator in accordance with changes in the load impedance, a feedbackcircuit for said oscillator, said feedback circuit including a variableinductance, said inductance being the primary of a transformer, andmeans for varying the current in the secondary of said transformerdirectly relative to said load impedance, whereby to vary the platecurrent of said oscillator, inversely relative to said changing loadimpedance.

13. An impedance matching device as recited in claim 12 in which themeans for varying the current in the secondary of the transformerincludes the plate resistance of an electron tube in circuit with saidtransformer, and means for varying the plate resistance of said tubeinversely in accordance with the changing load impedance.

14. In a high frequency generator supplying power to a heater, means forcontinuously matching the generator to a varying load impedancecomprising an oscillator, means for varying the plate voltage inaccordance with a change in load impedance, a transformer, a feedbackcircuit for said oscillator, said feedback circuit including aninductance which is the primary of said transformer, an electron tubehaving its plate circuit in series with the secondary of saidtransformer, means for deriving a potential varying 10 inverselyrelative to said load impedance and means for controlling the plateresistance of said electron tube by said last named potential.

15. In a high frequency generator supplying power to a heater, means forcontinuously matching the generator to a varying load impedance whilesupplying constant power thereto comprising an oscillator, means forderiving a high frequency. feedback voltage varying inversely relativeto said load-impedance, a feedback circuit for said oscillator, powersupply means for said oscillator, and means varying the voltage outputof said power supply inversely relative to said feedback voltage.

16. Impedance matching apparatus as recited in claim 15 in which themeans for deriving a potential proportional to said feedback voltagecomprises a closed circuit including a unilaterally conducting deviceand a resistive element across which to develop a direct currentpotential to control the operation of said power supply means.

17. In a high frequency generator supplying power to a heater, means forcontinuously matching the generator to a varying load impedancecomprising an oscillator, a high frequency feedback circuit for saidoscillator, said feedback circuit including a variable reactive element,means for varying the reactance of said reactive element inverselyrelative to the change in load impedance, power supply means for saidoscillator, said power supply means including an electronic tube havinga control element, means for deriving a potential proportional to saidfeedback voltage and means for applying said potential to the controlelement of said electronic tube to control the voltage output of saidpower supply means.

18. Impedance matching apparatus as set forth in claim 17 wherein themeans for applying the potential for the control element of theelectronic tube includes a phase shifting network.

19. In a high frequency generator supplying power to a heater, means forcontinuously matching the generator to a varying load impedancecomprising an oscillator, means for varying the plate voltage inaccordance with a change in load impedance, a transformer, a feedbackcircuit for said oscillator, said feedback circuit including aninductance which is the primary of said transformer, an electron tubehaving its plate circuit in series with the secondary of saidtransformer, means for deriving a potential varying in accordance withthe variation in plate voltage, and means for varying the plateresistance of said electron tube in proportion to said plate voltage.

20. In a high frequency generator supplying power to a heater, means forcontinuously matching the generator to a varying load impedancecomprising an oscillator having plate current and output voltageparameters, means for varying the oscillator plate current inverselyrelative to the changing load impedance, power supply means for saidoscillator providing a direct current plate potential, means fordeveloping a voltage proportional to the oscillation voltage of saidoscillator, and means for varying the output of said power supply inaccordance with said last named voltage.

21. In a high frequency generator supplying power to a heater, means forcontinuously matching the generator to a varying load impedanceComprising an oscillator, a high frequency feedback circuit for saidoscillator, power supply 11 means for said oscillator including anelectron discharge devige having a eohtrol element, means or der n a potia ropor onal 0 t e voltage of said feedback Qiffipliti, and means forappl ing said po n ial to't e c ntro 'e ement .of J

MITTELMANN.

Referenees Cited in the file of this patent UNITED STATES PATENTS Numb2,138,138 2,175,694 2,467,285 2,470,343

10 Number Name Date Bruckner Nov. 29, 938 n Jr. c 939 Young et a1 Apr.12, 1949 l Mittel mann May 1'7, 1949 FOBELGN PATENTS Country Date GreatBritain Mar. 7, 1935 Great Britain Dee) 2, 1935

